It is common in the rubber industry, such as the tire industry, to reinforce rubber compositions with particulate filler. Among the advantages of doing so, the particulate filler can bolster the modulus of the rubber composition. For example, silica has advantageously been employed as a filler. The use of silica filler within tire treads produces, among other advantages, improved wear.
While fillers offer advantages in rubber compositions, the presence of the filler impacts the dynamic properties of the rubber compositions. Namely, hysteretic loss increases with filler concentration. This can be disadvantageous, especially in tire treads, because hysteretic loss is inversely proportional to rolling resistance.
It is known that polymers can be modified with certain functionalities that react or interact with filler and thereby reduce hysteretic loss. This reaction or interaction between the polymer functionality and the filler particle is believed to reduce polymer loose ends and disassociate filler agglomerates. For example, it is known to functionalize polymer chains with silicon-containing functionalities that react or interact with, or can be hydrolyzed to form functionalities that react or interact with, the silica filler. While these functionalities have proven useful in reducing hysteretic loss, the presence of these functionalities can present processing issues.
To begin with, by reacting the polymer with functional compounds designed to react or interact with filler, the ability to otherwise react the polymer can be sacrificed. For example, it may be desirable to couple polymer chains to build molecular weight in order to facilitate polymer isolation and/or handling. Once the polymer chains have been reacted with a functionalizing agent, however, the ability to react the polymer with a coupling agent is precluded.
Also, the presence of a silicon-containing functionality, such as an alkoxysilane functionality, which can hydrolyze into silanol functionalities, serve as locations where the polymer chains can couple, especially over time. While, as suggested above, polymer coupling can be advantageous for polymer isolation, long-term growth in polymer molecular weight, also known as Mooney growth, is not desirable. Indeed, long-term Mooney growth can frustrate future processing of the polymer, which takes place when, for example, the polymer is employed in the production of tires.
Attempts have been made to alleviate this Mooney growth. For example, U.S. Pat. No. 5,659,056 teaches the addition of a stabilizing agent that does not react with the polymer functionality but instead serves to neutralize the bi-product lithium compounds that may be present from polymer initiators. At neutral pH, the Mooney viscosity jump is less severe. Alternatively, U.S. Pat. No. 6,279,632 teaches a method for stabilizing Mooney viscosity growth by treating these polymers with long-chain alcohols. And, U.S. Pat. No. 6,255,404 teaches a method for stabilizing Mooney viscosity growth by treating polymers with silicon-containing functionalities with alkyl alkoxysilanes.
While the foregoing approaches have been useful, the silicon-containing functionalities on many useful polymers have a very high affinity to undergo hydrolysis reactions with water (i.e. hydrolyze), and therefore there is a need to develop a more aggressive means to stabilize the polymers from Mooney viscosity growth.